Generally, an electric equipment of a switch board or the like is integrated with a number of electric wires and therefore, the electric wires are indicated with characters or signs in order to show kinds or functions of the respective electric wires. In that case, the character or the sign cannot directly be printed to the electric wire and therefore, the character or the sign is printed to a short tube and indicated by inserting the electric wire into the tube.
A tube is printed by a thermal transfer printer. That is, a long tube is passed between a platen roller and a printing head of the thermal transfer printer along with an ink ribbon, the tube is fed while rotating the platen roller, the printing head is partially heated, and ink of the ink ribbon is melted to thermally transcribe on a surface of the tube. After printing, the tube is cut in a predetermined length to use.
In order to feed the tube to the printing head, there is constructed a constitution of arranging a wrapping roller at a position opposed to the platen roller, passing the tube between the platen roller and the wrapping roller and feeding the tube to the printing head by rotating the platen roller. At this occasion, in order to firmly feed the tube, there is constructed a constitution in which also the wrapping roller is made to be soft (made of rubber) similar to the platen roller to thereby increase an area thereof brought into contact with the tube. Further, the tube is brought into a state of being deformed between the platen roller and the wrapping roller and is fed to the printing head under the state.
In this way, the tube is deformed between the wrapping roller and the platen roller immediately before printing and a surface of the deformed tube is printed. Further, since the printing head is formed in a planer shape and therefore, it is preferable that the deformed portion is as flat as possible.
However, the character printed on the surface of the tube is not necessarily excellent. When the printed character is observed in details, it has been found that the character seems to be blurred at a central portion thereof. Hence, as a result of investigating a cause thereof, according to the above-described failure in printing, when the tube is deformed, as shown by FIG. 4, although walls on both sides of a central portion a of a portion deformed between the wrapping roller 10 and the platen roller 11 are brought into contact with each other, both ends b thereof are produced with spaces on inner sides thereof since there is a limit in elasticity of the tube 5. Therefore, a section thereof is liable to be formed substantially in a “8”-like shape. Therefore, it has been found that the failure in printing is caused by printing the character while the central portion stays to be recessed. Particularly, as shown by FIG. 5, in deforming the tube 5 having a section formed with recessed and projected grooves at an inner face thereof, the tube 5 is further difficult to be a plane.